The present invention relates to a GMR bridge detector in which magnetoresistive resistors are interconnected in the form of a bridge to detect a magnetic field.
In a GMR bridge detector provided by the company Nonvolatile Inc., Eden Prairie, MN 55344, U.S. that is described as the NVS5B50 GMR Bridge Detector in a company brochure dated August 1995, four resistors made of GMR material are interconnected in a Wheatstone bridge to detect magnetic fields. In this known bridge detector, two of the four resistors are magnetically shielded by thick magnetic material to thus have fixed resistance reference values, and two further resistors change their resistance according to the applied magnetic field or rather the magnetic field to be measured. The change in the resistance value of these two resistors can lead to a voltage change of approx. 5-6% of the applied voltage. The two resistors exposed to the magnetic field that change their resistance are arranged between specially formed magnetic flux concentrators. In this manner, the detector is saturated for fields that are smaller than the saturation fields of the actual GMR material used. This known arrangement shows how difficult it is and what significant expense is required to manipulate the resistors of a bridge circuit so that they have different sensitivity to the external magnetic field and react accordingly.
GMR stands for Giant Magnetoresistive Ratio, i.e., the magnetoresistive properties of this material, i.e., its magnetoresistive effect, are considerable and significantly greater than in an ordinary magnetoresistive material. In an article entitled xe2x80x9cGiant Magnetoresistance at Low Fields in Discontinuous NiFexe2x80x94Ag Multilayer Thin Filmsxe2x80x9d by T. L. Hylton, K. R. Coffey, M. A. Parker, J. K. Howard that appeared in SCIENCE, Vol. 261, Aug. 20, 1993, pp. 1021-1024, various multilayer thin-film arrangements are described that include the material combination NiFe/Ag in respectively different layer thicknesses. In these materials, a significantly increased magnetoresistive sensitivity is observed through heating or tempering at (annealing) a specific temperature of about 330xc2x0 C. In the annealing process, discontinuous layer structures arise by way of which the GMR effect is supposed to be caused. The annealing is carried out through normal heating of the material and subsequent cooling down. Nothing is mentioned in this literature source regarding a specific arrangement of these materials in a certain application manner.
A method according to the present invention for manufacturing a GMR bridge detector has the advantage of significantly simplified and very easily controllable and meterable annealing of the resistors. The result is a significantly more cost-effective manufacturing process for GMR bridge detectors and associated with this, a more cost-effective component.
According to the present invention, this is achieved in that the magnetoresistive sensitivity of the individual resistors is produced through annealing, that the annealing of the resistors is performed through selective feeding of a current sufficient for attaining the temperature needed for annealing into the bridge connections and thus the resistors are provided with the property needed for the GMR effect.
According to a first embodiment of the method according to the present invention, each resistor is heated individually to the temperature sufficient for annealing by applying a voltage sufficient for annealing to its directly neighboring bridge connections and then cooled down thereafter.
According to a second embodiment of the method according to the present invention, it is provided that in each case two resistors lying in different bridge arms are annealed by feeding in current via in each case two directly neighboring bridge connections placed at the same potential.
In a third embodiment of the method according to the present invention, the individual resistors have in each case the same resistance value prior to annealing. Advantageously, the feeding in of the suitable current takes place once in the method according to the invention.
The method according to the present invention is suited in a particularly useful refinement to manufacture of GMR bridge detectors if as the material for the resistors a material from the class of discontinuous multilayer materials, particularly NiFe/Ag, is used in which the GMR property can be produced through annealing at a certain temperature.
In another embodiment of the present invention, a magnetoresistive bridge detector is created in which resistors are interconnected in the form of a bridge to detect a magnetic field. The resistors consist of a material that exhibits the GMR effect. The magnetoresistive sensitivity of the individual resistors is manufactured through annealing, and the annealing has taken place once through heating to the temperature necessary for annealing using controlled feeding of current into the bridge connections. In this magnetoresistive GMR bridge detector, as a material for the resistors, a material is used that exhibits after a single annealing process a different GMR effect than before, particularly a material in a multilayer arrangement of the material combination NiFe/Ag.